The present invention relates to a highly oriented permanent magnet such as a "wiggling" magnet used to pick up radiation from particle accelerators or one which is employed in an MRI (nuclear magnetic tomographic resonance imaging) device. More particularly, the present invention relates to a permanent magnet having the direction of magnetization inclined at a very small angle with respect to the line normal to a reference plane, as well as a process for producing such a permanent magnet.
Free electron lasers and particle accelerators such as synchrotrons have output radiation picked up by means of a plurality of permanent magnets disposed in an array. In those apparatus, a continuous array of permanent magnets called "wigglers" or "undulators" is disposed on either side of the channel of electron beams, with adjacent permanent magnets and those opposed to each other being arranged to have opposite polarity so that an alternating magnetic field is applied perpendicularly to the direction in which the electron beams travel. Some apparatus employ a "hybrid" system in which an array of permanent magnets are combined with yokes made of such as alloys as Permendur and Permalloy.
An example of "wiggler" array is shown in FIG. 5. Several tens of magnet pairs which are magnetized in such a way that fluxes come into and go out of the magnets perpendicularly to the planes ab which are defined by the longer side a and the shorter side b of the magnets which are arranged to present alternating N and S poles. Electron beams passing between two "wiggler" arrays are bent as they travel through the alternating magnetic field, with subsequent emission of radiation having a specified wavelength.
The permanent magnets used in the applications described above are required to have high magnetic characteristics and those which are made of anisotropic rare earth elements such as Sm-Co and Nd-Fe B systems are commonly employed to satisfy this requirement. Permanent magnets to be used as "wigglers" are generally designed to satisfy the relationship a.gtoreq.b&gt;c where a is the longer side or major axis of an individual magnet, b is the shorter side or minor axis of the magnet, and c is the thickness of the magnet. The requirement for permanent magnets that are to be used as "wigglers" in particle accelerators is particularly stringent in that the direction of magnetization should not be inclined with respect to the line normal to an installation reference plane at an angle exceeding 3 degrees, preferably not exceeding 2 degrees. If the angle of inclination exceeds 3 degrees, a component of magnetic field that is not perpendicular to the direction in which electron beams travel will develop and the resulting decrease in the effective component will cause problems such as variations in the bending of electron beams and hence the wavelength of output radiation. It is therefore required that the angle at which the direction of magnetization is inclined should be uniformly distributed in the plane ab of a permanent magnet and should not exceed 3 degrees, preferably 2 degrees.
The demand for constructing particle accelerators of a larger capacity is increasing today. To meet this need, large permanent magnets are fabricated by assembling a plurality of magnet blocks with an adhesive. However, the attempt to bond a plurality of magnet blocks with an adhesive to make a larger anisotropic permanent magnet involves the following problems. First, the adhesive layer between adjacent magnet blocks forms a magnetic gap and the resulting decrease in magnetic flux in that area causes unevenness in the overall magnetic characteristics, with subsequent deterioration in the performance of an apparatus that employs the magnet assembly. Second, when a large anisotropic permanent magnet is incorporated into a free electron laser or a particle accelerator, it is placed under high vacuum in an environment containing ultraviolet radiation, so there is high likelihood that the adhesive used to bond magnet blocks deteriorates as a result of destruction of the polymeric structure of the resin on account of an uv initiated photochemical reaction. Further, the procedure of assembling a plurality of magnet blocks by bonding them together with an adhesive is not only complicated but also time- o consuming and it has been difficult to supply products of consistent and uniform quality.
The process of fabricating permanent magnets consists of molding a magnet material and sintering the molding. A problem with this process, if it is employed to make a large anisotropic permanent magnet, is that the molded magnet material often warps due to shrinkage that occurs during sintering. Compared to small ones, large magnets tend to develop large cracks or extensive warps. This is due to the following two problems which are encountered in the method of achieving orientation in a magnetic field in the conventional mold. First, unevenness in the distribution of pressure in the molding will introduce unevenness in its density. Second, unevenness in the magnetic field for orientation in the mold will introduce unevenness in the degree of orientation achieved. It is worthwhile to consider the second problem in somewhat greater detail. To satisfy the requirements for strength and rigidity, the conventional mold often has a monolithic structure of ferromagnetic materials such as tool steels and at the edges of the molding cavity, magnetic fluxes tend to pass through the mold more easily than the molding which has a lower permeability than the mold. For the reasons described above, the conventional mold has not been suitable for use in making wiggling magnets by shaping in a magnetic field.
With a view to overcoming this bottleneck, a cold isostatic pressing method (abbreviated as CIP) has been proposed in JP-A-62-64498 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). This method employs an in-field wet rubber press comprising a nonmagnetic container, an upper and a lower punch that are made of a magnetic material and that are adapted to penetrate through said container for pressurizing in said container a powder provided as a molding material, two coils wound around the two punches to produce a magnetic field acting upon the powder charged between said two punches, and an orifice bored through the side wall of said container and through which water is supplied to exert hydrostatic pressure on the powder to be pressurized in said magnetic field. The drawing of JP-A-62-64498 illustrates the relationship between the intensity of X-ray diffraction at a (002) surface and the angle of inclination with respect to the direction in which the magnetic field is applied, and shows that comparatively improved orientation can be achieved by CIP.
The above-described method of using an in-field wet rubber press, however, has its own problems. First, it is essential for this method to use an upper and a lower punch made of a magnetic material but then, the pressurizing force exerted by the rubber press is not isostatic but lateral pressure will be added. Not only does this uneven application of pressures cause deformation of the molding at its edges but also the angle at which the direction of magnetization is inclined will be affected. Second, the mold is required to have sufficient strength to withstand the pressure exerted by CIP. Third, sufficient electrical insulation must be provided to permit coils to be installed within the CIP apparatus. All of these factors present considerable difficulty from both technical and safety viewpoints.
Further, none of the permanent magnets fabricated by this method have yet satisfied the already-described requirements for "wigglers" in particle accelerators. This is because the application of the invention described in JP-A-62-64498 is limited in practice to a method commonly referred to as "longitudinal magnetic field pressing" in which the pressing direction is parallel to the direction in which a magnetic field is applied and there is a certain limit on the improvement that can be achieved in the degree of orientation.
The magnetic particles of which rare earth based permanent magnets are made are generally flat and their longitudinal direction substantially coincides with the easy axis of magnetization, and when the magnetic particles loaded into the mold are pressurized, they tend to orient in such a way that their longitudinal direction is perpendicular to the direction in which they are compressed. Therefore, if one wants to fabricate a permanent magnet of high performance, it is preferred to employ a method called "lateral magnetic field pressing" in which molding is effected in a magnetic field that is applied in a direction perpendicular to the pressing direction because this contributes to an improvement in the degree of orientation.
Under the circumstances described above, it has been strongly desired to develop a permanent magnet in which the angle of inclination of magnetizing direction is very small and uniformly distributed and which has previously been considered difficult to fabricate by shaping in a magnetic field in the prior art mold. A need has also been recognized for producing such a permanent magnet by a method that utilizes the advantages of both the lateral magnetic field pressing and CIP processes.